Hello. I'm not sure I know! I recall seeing them with the serial number only on the frame...as best I remember, but understand that FN is "notorious" for variations both unannounced and not cataloged. One run of Mk III pistols evidently intended for the US has serial numbers on the frame only. The next batch may have them on the barrel as well as in some countries, the serial number must be on the slide or barrel rather than the frame. I'm not a historian, but I do think it might be possible that T guns can be found with serial numbers exclusively on the frame and others having them on the barrel and/or slide as well! Hopefully someone with more information that I have will respond, but for now, I'd consider the possibility that serial numbers may be multi-placed on some T guns and not on others.
Here is a little more information on the T guns from another site: